1. Field of the Invention
The present invention relates to a displacement control valve incorporated in variable displacement compressors that are used in vehicle air conditioners. More particularly, the present invention relates to a displacement control valve that controls the flow rate of refrigerant gas between discharge and crank chambers.
2. Description of the Related Art
A typical variable displacement compressor has a cam plate that is tiltably supported on a drive shaft. The inclination of the cam plate is controlled based on the difference between the pressure in a crank chamber and the pressure in cylinder bores. The stroke of each piston is varied by the inclination of the cam plate. Accordingly, the displacement of the compressor is varied and determined by the stroke of each piston. The compressor is provided with a discharge chamber and a crank chamber that are connected by a supply passage. A displacement control valve is located in the supply passage. The displacement control valve controls the flow rate of refrigerant gas from the discharge chamber to the crank chamber, thereby controlling the pressure in the crank chamber. Accordingly, the difference between the pressure in the crank chamber and the pressure in the cylinder bores is controlled by the control valve.
Japanese Unexamined Patent Publication No 6-346845 discloses such a displacement control valve used in a variable displacement compressor. As shown in FIG. 6, a control valve 101 includes a housing 102 and a housing 103 that are secured to each other. The solenoid 102 includes a fixed steel core 105, a steel plunger 106 and a coil 104. The plunger 106 moves closer to and away from the fixed core 105. The coil 104 is wound around the plunger 106 and the fixed core 105. A first spring 107 extends between the fixed core 105 and the plunger 106.
A valve chamber 108 and a pressure sensing chamber 115 are defined at the upper portion and at the lower portion of the hosing 103, respectively. The housing 103 has a first port 110, a second port 111 and a third port 112. The valve chamber 108 is connected to the discharge pressure area of the compressor by the first port 110, which opens in the ceiling of the valve chamber 108. The valve chamber 108 is also connected to the crank chamber in the compressor by a valve hole 114 formed in the bottom of the valve chamber 108, the third port 112 and the supply passage. The pressure sensing chamber 115 is connected to the suction pressure area of the compressor by the second port 111. The first port 110, the valve chamber 108, the valve hole 114 and the third port 112 constitute a part of the supply passage. A valve body 109 is provided in the valve chamber 108. A second spring 113 provided in the valve chamber 108 urges the valve body 109 in a direction closing the valve hole 114. A bellows 116 is provided in the pressure sensing chamber 115.
Suction pressure Ps in the suction pressure area of the compressor is introduced to the pressure sensing chamber 115 by the second port 111. The bellows 116 in the pressure sensing chamber 115 is expanded and collapsed in accordance with the suction pressure. The bellows 116 is attached to the plunger 106 in the solenoid 102. A rod 117 is secured to the top of the bellows 116. The distal end of the rod 117 contacts with the valve body 109 in the valve chamber 108. Changes in the length of the bellows 116 are transmitted to the valve body 109 by the rod 117. The valve body 109 opens and closes the valve hole 114, accordingly. In other words, the valve body 109 opens and closes the supply passage, which connects the discharge pressure area with the crank chamber, in accordance with changes in the suction pressure Ps in the pressure sensing chamber 115.
The solenoid 104 is excited and de-excited by an external control computer (not shown). The computer is connected to various devices including an air conditioner starting switch, an engine speed sensor, a temperature sensor for detecting the temperature of the evaporator in the external refrigerant circuit, a compartment temperature sensor, and a temperature adjuster. A passenger sets a target compartment temperature by the temperature adjuster. The computer inputs data relating to the ON/OFF state of the starting switch, an engine speed, an evaporator temperature, a compartment temperature, and a target compartment temperature. Based on the inputted data, the computer excites and de-excites the solenoid 104.
When the solenoid 104 is excited, the plunger 106 is attracted to the fixed core 105 against the force of the first spring 107. This movement of the plunger 106 is transmitted to the valve body 109 by the bellows 116 and the rod 117, thereby moving the valve body 109 toward the valve hole 114. In this state, if suction pressure Ps is high, that is, if the cooling load is great, the bellows 116 collapses and pulls the valve body 109. This decreases the opening area of the valve hole 114. Contrarily, if the suction pressure Ps is low, that is, if the cooling load is small, the bellows 116 expands and pushes the valve body 109. This increases the opening area of the valve hole 114.
When the solenoid 104 is de-excited, there is no magnetic attractive force between the plunger 106 and the fixed core 105. The plunger 106 is thus moved by the force of the first spring 107 away from the fixed core 105. This movement of the plunger 106 is transmitted to the valve body 109 by the bellows 116 and the rod 117, thereby moving the valve body 109 away from the valve hole 114. This maximizes the opening area of the valve hole 114.
The valve body 109 and the rod 117 are constantly in contact with each other by the forces of the first spring 107 and the second spring 113. This permits an integral movement of the valve body 109, the rod 117 and the bellows 116.
The heat exchange capacity of the evaporator in the external refrigerant circuit is extremely low, for example, when the outside temperature is high and the vehicle speed is low. In this case, if the compressor is operating at the maximum displacement, the discharge pressure Pd in the discharge pressure area becomes extremely high. The suction pressure Ps, which is introduced to the pressure sensing chamber 115, also becomes extremely high. This collapses the bellows 116. If the starting switch is turned off, t computer de-excites the solenoid 104 to minimize the compressor's displacement. In this state, the valve body 109 needs to be at such a position that the opening area of the valve hole 114 is maximized. However, the high suction pressure Ps in the pressure sensing chamber 115 keeps the bellows 116 collapsed. Further, the valve body 109 is moved integrally with the bellows 116. Therefore, it is hard to keep the valve body 109 at a position for maximizing the opening area of the valve hole 114.